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    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
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    <title>Document</title>
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<body>
    <script>
        //作用域链：函数使用变量的时候，采用最近原则。
    // 案例1 ：结果是几
    var num = 456;
    function f1(){
        var num = 123;
        function f2(){
            var num = 0;
            console.log(num);//站在目标出发，一层一层的往外查找
        }
        f2();
    }
    f1();
   // 案例2 ：结果是几？
   var a = 1;

function fn1() {
    var a = 2;
    var b = '22';
    fn2();

    function fn2() {
        var a = 3;
        fn3();

        function fn3() {
            var a = 4;
            console.log(a); //a的值 ?
            console.log(b); //b的值 ?
        }
    }
}
fn1();
    </script>
</body>
</html>